Blocked holes boost light on the nanoscale

PRINCETON, N.J. – Capping a hole should block light transmission through that hole, right? Not on the nanoscale. In fact, placing a metal cap over a small hole in a metal film does not stop light from passing through the hole, but rather enhances its transmission.

This finding by scientists at Princeton University could have significant implications and various uses, particularly in optical instrumentation.

In an example of the “twists” in physics that can occur at very small scales, electrical engineer Stephen Chou and his colleagues made an array of tiny holes in a thin metal film, then blocked each hole with an opaque metal cap. When they shined light into the holes, they found that as much as 70 percent more light came through when the holes were blocked than when they were open.

These electron microscope images show an experiment demonstrating that blocking a hole in a thin metal film could cause more light to pass through the hole than leaving it unblocked does. The top image shows an array of holes with gold caps, each of which is 40 percent bigger than the hole on which it sits. The bottom image shows a cross-section view of one hole with the cap sitting on top. Surprisingly, a hole covered with the cap allows more light to be transmitted through the film than a hole without the cap. Courtesy of Stephen Chou/Princeton University.
Chou said that the result might require scientists and engineers to rethink techniques they use to block all light transmission. In very sensitive optical instruments – for example, spectrometers, telescopes, microscopes and other optical detectors – it is common to coat a metal film onto glass with the intention of blocking light. Dust particles, which are unavoidable in metal film deposition, inevitably create tiny holes in the metal film, but these holes have been assumed to be harmless because the dust particles become capped and surrounded by metal, which was thought to block the light completely.

Chou said that this assumption is not correct; rather, the plug greatly enhances the leakage. He explained that in his own field of nanotechnology, light often is used to carve ultrasmall patterns in silicon or other materials. Thin metal film patterns on a glass plate serve as a mask, directing light through certain locations and blocking others. Given the new finding, engineers ought to examine whether the mask blocks the light as expected, Chou said.

Chou said the metal disk acts as a sort of antenna that picks up and radiates electromagnetic waves. In this case, the disks pick up light from one side of the hole and radiate it to the opposite side. The waves travel along the surface of the metal and leap from the hole to the cap or vice versa, depending upon which way the light is traveling.